A backlight unit of the general edge light type for liquid-crystal displays is constituted of a lightguide plate having dots for light diffusion printed on the back side thereof, a light source (e.g., a cold cathode fluorescent tube or LEDs) disposed on one or each side of the lightguide plate, a light diffusing film superposed on the lightguide plate, a lens film (prism film) superposed on the light diffusing sheet, etc.
One role of the light diffusing film in the backlight unit is to diffuse the light which has passed through the lightguide plate and thereby prevent the dots on the back side of the lightguide plate and bright lines of the light source from being visually recognized on the screen of the liquid-crystal display. Another roll of the light diffusing film is to lead the light which has passed through the lightguide plate and has a large brightness peak angle (the angle at which luminance has a peak; it means an angle with the front direction) to the lens film while converting it into diffused light having a smaller brightness peak angle than that in order that the diffused light might be brought to the front direction (the direction perpendicular to the screen of the liquid-crystal display) by the lens film to further heighten luminance.
Known as a light diffusing film which plays such roles is a film (sheet) in which projections of a rectangular pyramid shape having a vertex inclined to either of the left and right have been arranged lengthwise/crosswise on the light emission side (patent document 1). This light diffusing film (sheet) has a constitution in which each projection of a rectangular pyramid shape has a difference in bevel between the left-side and right-side inclined faces, and is intended to thereby lead the light which has passed through the lightguide plate and has a large brightness peak angle to the lens film while converting it into diffused light having a smaller brightness peak angle than that.
However, this light diffusing film (sheet) having projections of a rectangular pyramid shape is difficult to continuously produce by forming a film by melt extrusion and embossing the film with an embossing roll or the like. Practically, products of this light diffusing film must be produced one by one by injection molding, hot-press molding, or the like. There have hence been problems that the light diffusing film has poor productivity and an increased cost and that it is not easy to produce thin films having a thickness of 150 μm or smaller. In addition, the formation of projections of a rectangular pyramid shape regularly arranged lengthwise/crosswise as in this light diffusing film (sheet) has had problems that marring is apt to occur because of the projections and this is causative of defects and that the projections may cause a moiré or interference fringe, resulting in luminance unevenness.
Patent Document 1: Japanese Patent No. 2948796